Kassie Rose, 30 years old, faces a frightening prospect: if a genetic coin toss fails to go her way, she could lose her mind within a decade or two. A mutation that causes Alz­heimer’s disease runs in her family, the DeMoes of North Dakota. The odds of any DeMoe harboring the mutation are 50–50, and if the mutation is present, the chances of developing early-onset Alzheimer’s—the type that erodes memory before age 65—are 100 percent.

Five of the six DeMoe siblings—Rose’s father and her aunts and uncles—have the mutation. One man is in a nursing home in his mid-50s; a second, younger, is on his way. A sister in her late 40s is already noticing her first symptoms. The next generation is tortured with the decision of whether to get tested. Rose, for now, chooses not to know. After all, she is unlikely to benefit much from the information: Alz­heimer’s remains incurable and, largely, unpreventable as well.